The long-term objective of the proposed research is to afford understanding of the biochemical mechanisms underlying the contractile properties of the heart. This objective is being approached through study of calcium fluxes across isolated sarcoplasmic reticulum membranes which in the intact heart play a central role in controlling the availability of activator calcium to the contractile proteins. The rapid kinetics of ATP dependent calcium transport will be studied in both cardiac and skeletal muscle sarcoplasmic reticulum in order to determine the partial reactions and allow identification of rate limiting steps that may be affected by hormones, cellular metabolites, and pharmacologic agents. The regulation of such rate limiting steps in calcium transport by cyclic AMP-dependent protein kinase and phosphodiesterase modulator protein will be investigated. Calcium release from calcium loaded vesicles will also be studied in an attempt to elucidate the mechanism of Ca 2 ion triggered calcium release observed by other investigators in skinned fibers. The role of the calcium pump in the release process will be explored as will be the possible existence of a transmembrane potential difference. The role of the 22,000 dalton phosphoprotein in cardiac SR membrane function will be studied by immunochemical techniques.